Most of the image projection apparatus for producing colored images on a screen only comprise one image panel: In order to form a colored image the panel needs to be sequentially illuminated with light beams of different colors. If the sequence is displayed fast enough the human eye is not able to timely resolve the resulting color image sequence and results in the impression of a colored image. One method to realize the required sequential illumination is to place into the illumination path dichroic filter segments secured on a rotatable disc shaped carrier. This forms a color disc which is typically connected to a motor. The assembly of such a color disc and the motor is called color wheel. Since the rotation of such a color disc needs to be quite fast, balancing of such a color disc becomes an issue.
In a first approximation the color disc can be described as a rigid disc shaped rotor. The state of the art solution to balance a disc shaped rotor is to add or to remove balancing weight separated from the axis. The straight forward way to perform such a balancing is to drill holes and thereby remove material.
To perform material removal, the color wheel is attached to a balancing machine which has a fixture to hold it. The color wheel is spun up and the machine is allowed to detect the residual imbalance of the color disc. It then outputs a mass value and angular location to correct for the imbalance. An operator then drills out the appropriate amount of material based on experience. The process is repeated until the color wheel is balanced to a specific level.
To perform material addition, a similar process is performed, but for imbalance correction, material such as lead or adhesive can be added to the color disc. Material addition is typically 180° from a material removal imbalance location.
In U.S. Pat. No. 6,747,803 one or more annular grooves are disposed on the external surface of the disc shaped carrier. This allows one to add a balancing substance into the annular grooves to balance the color wheel.
One of the difficulties is to find a method of how to accurately balance the wheel. In JP 2004325721 a color wheel is pinched and held between a support table and a pressing plate and driven with a motor. The pressing plate is provided with balance adjustment ribs and recessed parts. The balance adjustment ribs are so arranged to be dispersing in circumferential direction and extending in radial direction. The recessed parts are provided at positions corresponding to the balance adjustment ribs and approach to the outer peripheral edge of the pressing plate so that a space is formed between respective balance adjustment ribs and the pressing plate. The balance adjustment ribs are cut into a necessary length from the outer peripheral side by inserting such a multipurpose cutting tool as a nipper into the recessed parts.